There are various ways of manufacturing a Rogowski coil. A classical way involves winding the coil wire around a flexible (non magnetic) core. Another way involves “winding” the turns of wire around a hard core, such as a PCB. Thus, if both faces of the plate are used, each face has part of the circuit of each turn. In other words, the coil is implemented by means of metal deposits on each one of the board's two faces, extending along radii of the PCB. The result is a helical coil, whose cross-section is determined by the width of the coil and the thickness of the PCB.
One example of a Rogowski coil implemented on a PCB is the one disclosed in US patent document U.S. Pat. No. 5,461,309, which reveals a current measuring device based on a Rogowski coil implemented on a printed circuit board. The Rogowski coil has the shape of a flat disk, on which the coil is implemented by means of straight metal deposits on each side of the plate, along radii whose projections intersect in the central point of the coil. The electrical connections between the radii of one side and those of the opposite side are achieved by means of holes or paths that pass through the thickness of the board. These return coils in two different directions provide immunity against fields and currents that are external to the current to be measured. These coils are practically flat, which enormously facilitates their design and manufacture on PCB.
The type of Rogowski coil described for example in the preceding patent, is based on a flat circular structure (more specifically, ring-shaped) on a rigid PCB, wherein each winding (or turn of the coil) is positioned in a direction that is perpendicular to the conductor whose current is to be measured, as shown for example in FIG. 1 of U.S. Pat. No. 5,461,309. Consequently, these coils have a short length (measured in the direction of the conductor; in other words, their length coincides with the thickness of the PCB), have an inner diameter that is limited by the conductor and have an outer diameter that is conditioned by the coil's design. In other words, taking into account the limited freedom afforded by the thickness of the PCB and the inner diameter, the section of each winding is limited by the outer diameter of the PCB, which could be limited in the installation, thereby also limiting the coil section and its sensitivity for measuring the current.
US patent document U.S. Pat. No. 7,825,763B2 proposes an alternative design for a current transformer, wherein the transformer's secondary is formed by a complete tubular ring made up of four identical quarters of PCB, fixed to an annular frame. Each quarter of PCB is curved until completing an angular portion of a tubular ring whose axis of symmetry coincides with the axis of the transformer's primary conductor.
However, the above design requires several parts of PCB joined to each other mechanically and electrically, rendering manufacture more difficult and adding points of weakness to the design.